The operation of power grids is defined by certain rated operating parameters, normally voltage and frequency. The existence of imbalances between generated power and consumed power at a given time causes deviations in grid operating frequency. In particular, when generated power exceeds consumed power, grid frequency increases above its rated value. If, on the contrary, generated power is less than consumed power, frequency decreases in relation to its rated value. If said deviations are not quickly rectified, the disconnection of large grid areas may become necessary.
For the purpose of collaborating in the limitation of power grid frequency, conventional generating plants such as thermal or nuclear generating plants have means to, based on power grid operator demand, increase or decrease the active power generated in accordance with power grid conditions at a given time. To date, distributed generation plants (for example, those based on renewable energy such as wind or solar energy) have not been required to collaborate in grid stability. However, in view of the spectacular increase in installed power in this type of generating plants in recent years, the active power control requirements imposed by the grid operator are extending to distributed generation plants.
Grid operator requirements in terms of active power variation in response to deviations in frequency towards distributed generation plants vary as, depending on the type of power grid (higher or lower power rating) in a given area, the grid operator establishes more or less stringent requirements for the generators connected to said grid. For example, the maximum power limits established for primary frequency regulation may range between 1.5% and 100% of rated power. The response time for this also varies greatly.
At present, two basic types of control structures are used to adapt the active power generated to grid frequency. A first control structure used in wind farms is based on the local control of the active power generated by each individual wind turbine. An example of this type of structure is U.S. Pat. No. 6,891,281, wherein each wind turbine has a local controller that limits its power output in accordance with grid frequency, regardless of the power output of other turbines. Said controller has the same characteristics in all the wind turbines of the wind farm, in such a manner that, for the same wind and frequency conditions, power variation will be identical in all the wind turbines. However, this control strategy has the drawback that there is no supervision to ensure that the overall efficiency of the wind turbine system is adequate, due to which farm-level errors could occur in the efficiency required by the grid operator. Additionally, depending on the operator's requirements, the variation in active power of each wind turbine can be so small, for example a variation of barely a few kilowatts, that the wind turbine power control systems are unable to guarantee sufficient accuracy, for example due to the existence of dirt on the blade surfaces, speed and power metering device tolerances, etc. At global farm level, this can result in nonfulfilment of the grid operator's requirements.
The second known control structure is based on the centralised control of the wind farm, i.e. a central control unit sends the individual power commands to each wind turbine in real time in accordance with the grid frequency measurement. An example of this strategy is U.S. Pat. No. 7,372,173, where the central control unit measures grid frequency and, when it detects the presence of an error therein with regard to the reference value, sends the necessary active power commands to each wind turbine in real time to modify wind farm power output in accordance with the grid operator's requirements. These systems achieve a coordinated response from the farm, eliminating errors in wind turbine response. However, due to farm control cycles and delays in communications between the central control and the wind turbines, the response speed of the centralised systems is much slower than that of systems based on a local controller in each wind turbine. The delay between the moment in which a grid event occurs (such as for example a deviation in frequency) and the moment in which the wind turbines start responding to the farm control commands usually exceeds 100 ms. This can represent a serious drawback, as the grid operator occasionally requires faster variation in the active power generated by a wind farm.